Ansible is a pig that does many things, very few of them right or well. Going by your logic, why use a dedicated IaC tool like Terraform or Pulumi if you can just force Ansible to kind of do it ?

New tools covering a specific niche, even if that niche is already covered by a bigger, more established tool, are frequent. Not everyone needs or wants the kitchen sink approach. If you're using only CloudFormation to manage AWS, why would you force misery upon yourself to use Ansible just for AWS auth?

* I slightly exaggerate, but i genuinely consider Ansible to be the worst CM tool of the "modern" ones (so excluding CFEngine) at any sort of scale/enterprise. It has so many things wrong it's baffling for me people are removing existing Puppet, Chef, Salt envs to replace them with Ansible. It's awesome for homelabs or similar though.